Sedimentology and geochemistry of sediments from the Sunda Shelf

The Sunda Shelf represents one of the largest tropical shelf areas in the world. Its broad extension, the very low gradient, and the tectonical stability offer excellent conditions for a reconstruction of the exposure and flooding history during the last glacial cycle. A high fluvial input dominated this siliciclastic deposystem over long time intervals.
The proximal part of the transect of the research cruise Sonne-115 covers the riverine structures of the ancient North Sunda River ('Molengraaff' River) from -70 to -126 m modern water depth. The aim of the study presented here was to analyse the stratigraphic architecture of the Sunda Shelf during the last 50 kyr, to develop a high-resolution sea-level curve, as well as to reconstruct the flooding history of this part of the world.
The facies types examined in 36 sediment cores were classified by sedimentological and geochemical methods. 80 AMS-14C datings and additional fractionation measurement led to a high-resolution age determination. A shallow-seismic survey along and crosswise to the transect illustrated large- to medium-scale sediment structures.
Eleven facies types were identified and related to the paleo-shoreline. They extend from terrestrial and coastline deposits to full-marine environments and were classified into five groups along the transect, which contain distinct local facies successions.
The sea-level rise from 21 to 11 cal. kyr BP was determined by dating of plant remnants from the intertidal zone, following the retrograde migrating paleo-coastline. During lowstand, sea-level dropped at about -120 m. A conspicuous phase of the subsequent rise between 14.6 and 14.3 cal. kyr BP corresponding to meltwater pulse 1A was documented the first time and indicated as much as 16 m.
In response to the sea-level changes during the last 50 kyr, characteristic processes formed the deposits and changed drastically several times until the terrigenous supply starved at about 11 kyr BP. Most remarkable were incised valley fills, which preserved deposits of the transgressive phase. A sequence-stratigraphic interpretation classified the deposits into three systems tracts and indicates, that three major factors controlled the depositional history: sediment input, physiography, and sea-level fluctuations. The specific modification of these factors in comparison to the general model was related to the asymmetry of the sea-level fall and rise and the extreme morphological conditions on the Sunda Shelf. Nevertheless, hiatuses of several thousand years and massive erosion activity were almost always predominantly affecting and strongly reducing the original sediment thickness.

巽他陆架（Sunda Shelf）是全球规模最大的热带陆架区域之一。其广阔的展布范围、极低的地形坡度与构造稳定性，为重建末次冰期旋回期间的暴露与淹没历史提供了绝佳条件。长期以来，高河流输入主导了该区域硅质碎屑沉积体系的演化。
索恩（Sonne）-115科考航次断面的近端区域，覆盖了现代水深-70至-126米区间内的古北巽他河（莫伦格拉夫河，Molengraaff River）河道构造。本研究旨在解析过去5万年以来巽他陆架的地层结构，构建高分辨率海平面曲线，并重建该区域的淹没历史。
本次研究对36个沉积岩芯中的沉积相类型开展了沉积学与地球化学方法分类。通过80次加速器质谱碳-14（AMS-14C）测年与额外的分馏测量，实现了高精度年代标定。沿断面及跨断面开展的浅地层地震勘探揭示了中至大型沉积构造。
研究共识别出11种与古海岸线（paleo-shoreline）相关的沉积相类型，其沉积环境涵盖陆相、海岸带至全海相，并沿断面被划分为5个具有独特局部相序的组合单元。
通过对潮间带植物残体的测年，明确了21至11校准千年BP（cal. kyr BP）期间的海平面上升过程，该过程对应向陆迁移的古海岸线。在低水位期，海平面降至约-120米。本研究首次记录了14.6至14.3校准千年BP期间与融水脉冲1A（meltwater pulse 1A）对应的显著海平面上升阶段，其海平面上升幅度可达16米。
受过去5万年海平面变化的影响，沉积作用形成了一系列特征性过程，并在约11校准千年BP陆源供给枯竭前发生了多次剧烈转变。其中最为显著的是下切谷充填，其保存了海侵期的沉积记录。层序地层学解释将沉积记录划分为3个体系域，并指出三大主控因素控制了沉积演化历史：沉积物输入、地形地貌与海平面波动。相较于经典层序地层模式，本研究区的主控因素变化与海平面升降的不对称性及巽他陆架极端的地形条件密切相关。尽管如此，数千年的沉积间断与强烈的侵蚀作用始终广泛发育，大幅降低了原始沉积厚度。